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A series of 0.7 μm thick GaN layers have been grown by solid-source rf-plasma assisted molecular beam epitaxy on sapphire (0001) substrates with the addition of 0.10%–3.30% Al. The Al concentration was determined by secondary ion-mass spectrometry and Auger-electron spectroscopy, while the layer quality was assessed by photoluminescence, Hall effect measurement, and high-resolution scanning electron microscopy. Microscopy revealed a surface roughness varying with Al content. The smallest surface roughness was obtained at 0.10% and 3.30% Al. Low-temperature photoluminescence revealed dominating peaks attributed to the neutral donor-bound exciton. Its energy increased slightly with Al concentration, which established a correlation between the Al concentration and the band gap. The surface morphology, the corresponding optical and electrical properties, showed a clear improvement of the GaN layer quality for the range of 0.10%–0.17% Al content. © 2001 American Institute of Physics.